Method of melting metal powder in vacuo



Nov. 20, 1956 D. WROUGHTON 2,771,357

METHOD OF MELTiNG METAL POWDER IN VACUO Filed July 27, 1944 2 Sheets-Sheet 1 s K my? V g: 1 :m

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FI/ZZ 7 7 L \g\ X \i 5 INVENTOR ATTORNEY Nov. 20, 1956 D. WROUGHTON 2,771,357

METHOD OF MELTING METAL POWDER m VACUO Filed July 27', 1944 2 Sheets-Sheet 2 22 '4 BY a5 24 Q. M ATTORNEY METHOD OF MELT-INGNETAL POWDER 1N VACUO Application July 27, 1944, Serial No. 546,913

Ill-Claims. (Cl. 75-84.1)

This invention relates to a method for refining uranium and, more particularly, to a method for melting and casting uranium in substantially pure coalescent form.

In a U. S. patent issued to William C. Lilliendah'l et al., No. 2,690,421, September 28, 1954, a method is described whereby uranium may be prepared in the form of a compressed coherent powder. Briefly, the method thereof includes the electrolysis of potassium uranous fluoride, KUFs, uranium tetrachloride, UCl4, or uranium tetrafiuoride, UF4, in a fused salt bath of sodium and calcium chlorides in which uranium is deposited on the cathode in the form of a powder, the separation of the uranium powder from soluble impurities by washing with water and acid, and finally the compression of the uranium powder into a slug or button while protecting it from oxidation. By heating the uranium button to sintering temperature in a high vacuum as described in United States Letters Patent No. l,8l4,7l9 to J. W. Mai-den et al., many vaporizable impurities may be eliminated from the button, but non-vaporized impurities remain in the sintered mass. Such impurities cannot be tolerated in certain uses of uranium.

In a copending application of George Meister, Serial No. 533,132, filed April 28, 1944, now U. S. Patent No. 2,756,138, issued July 24, 1956, there is disclosed a method of and apparatus for separating uranium metal from the impurities contained in uranium buttons, such as those resulting from the above described U. S. patent of Lilliendahl et al., No. 2,690,421. According to the invention of Meister, disclosed in Serial No. 533,112, an impure uranium button is supported on a strainer or bridge member that is in turn supported in and near the bottom of a crucible having a funnel shaped bottom portion with a hole or drain therethrough. The crucible rests on the upper open edge of a mold with the funnel portion of the crucible extending into the upper portion of the mold. This arrangement is suitably supported in a chamber that may be evacuated, and electrical, high frequency, induction means are provided to heat and melt the button of uranium. The chamber also is provided with means for the introduction of a non-oxidizing gas such as helium or argon. As the uranium button is heated up to its melting temperature by means of the induction heater, vaporizab'le impurities that may be present in small amounts are liberated, and the non-vaporizable impurities are retained on the strainer in the form of a slag, while the uranium metal flows down through the strainer and the funnel opening in the bottom of the crucible and into the mold there below.

A difficulty encountered in separating refined uranium from non-vaporizable impurities by the above described procedure arises from the adherence of a large portion of the molten metal to the slag residue. This causes a serious inefficiency of separation and loss of precious metal. The difficulty is believed to be due in part to surface tension of the molten metal that may be increased materially by an oxide skin that forms around the mass of metal and slag-like impurities, and that tends to prenited States Patent vent the metal from flowing away from the impurities. The difiiculty is also believed to be due in part to inefiicient heating toward the end of the melting process. As the melting proceeds, the fused metal gradually flows from the button in droplets leaving the button in substantially its original form, but growing smaller and smaller in size. This reduces the efficiency of the induction heating means as the proportion of non-conducting slag increases, and it becomes impossible to maintain the required temperature of the mass long enough to complete the separation. As a result of these two unfavorable conditions, a substantial amount of metal remains with the slag. This may often amount to as much as 50 percent of the metal contained in the original button.

In his above mentioned application, Serial No. 533,112, Meister disclosed a modification of the induction heating means whereby the second of these unfavorable conditions is largely eliminated, said modification consisting of an electrical conducting coil placed inside the evacuated chamber and around and closely adjacent the electrically non-conducting crucible containing the button to be melted. This coil is heated by the induction heater and radiates its heat to the button sufficiently to maintain the button at, or somewhat above, the melting temperature of uranium after direct heating of the button by induction has become too inefiicien't. The last mentioned coil may likewise assist in the initial heating of very impure buttons until fusion of the metal contained therein increases their conductivity enough for efficient induction heating to begin. However, presumably because of the surface tension of the molten metal itself, the formation of an oxide film enveloping it, and adherence of the metal to the impurities, a substantial portion of the uranium is still left with the slag, the loss sometimes mounting to as much as 40 percent of the original content of the button.

An object of the present invention is to provide a method of effecting more complete recovery of refined uranium than has been possible heretofore.

A further object of the invention is to speed up the flow of molten metal from the slag during the final stage of the process in order to effect the production of better castings than have been obtainable from the methods described above. I

Additional objectives will be apparent from the following description and from the drawings in which:

Fig. 1 is an elevation, partly in section, of a preferred form of apparatus set up for carrying out the first portion of my process;

Fig. 2 is a sectional elevation of a portion of Fig. l, somewhat enlarged, at a later stage in the process; and

Fig. 3 is a sectional elevation of an arrangement of apparatus to be employed in place of certain parts shown in Fig. 1 for carrying out the second portion of my process.

The preferred form of apparatus shown in Fig. l is fully assembled and ready for operation. An impure uranium button 1 rests on the bottom of a crucible 2, supported by a massive block of tungsten 3 on top of a support 4. These parts are all mounted on a bronze plate 5 that provides a base for the entire assembly.

The crucible and supports are enclosed by an envelope 6 constructed of a refractory glass, such as Vycor, a 96 percent silica glass having a softening point of about 1500 C. Water-cooled Pyrex glass may be used but has a shorter life than Vycor. This is due to the 'fact that Vycor glass withstands acids used to remove condensed impurities better than Pyrex glass. The envelope 6 should be of such a height that it extends well above the crucible 2, thus providing a space above the crucible in which vaporized impurities may be dispersed and cooled at points where they will not recombine with,

or contaminate the metal being treated. The inner upper wall of the Vycor envelope 6 provides a surface on which vaporized impurities may be condensed at a suitable distance from the hot metal. An induction heating coil 20 connected with a suitable source of high frequency current extends around the envelope 6 and is mounted on a sleeve 23 slidably mounted for movement up and down a vertical rod or standard 22. A pair of pipes 24 and 25, in open communication with the interior of the envelope 6, are connected to a vacuum pump and to a source of inert gas, respectively, neither of which is shown in the drawing.

In order to avoid contamination of the uranium, and in order that the apparatus will withstand the tempera? tures employed, great care must be exercised in choosing the materials of which the apparatus is made. All materials must be inert with respect to uranium and highly refractory. All parts of the apparatus that are directly contacted by the molten metal are preferably made of beryllia or thoria, as these refractory oxides are inert with respect to molten uranium. Carbon, graphite, silicon carbide, or alumina or such materials lined with beryllia or thoria may be used for these parts. The support 4 is preferably constructed of silica or alumina. Before use, each piece of the apparatus contained in the envelope 6, and the envelope itself, should be prebaked at a temperature above the melting point of uranium in it will not readily oxidize in air, the envelope 6 is removed, andthe crucible 2 (and its contents) and the block 3 are replaced by the crucible 13 containing a strainer or bridge 14 resting near the bottom thereof, the mold 15, and the supporting block 16 of a metal such as tungsten, all in the relation shown in Fig. 3. The button is removed from the crucible 2 in its solidified state (designated 12 in Fig. 3) and placed on the bridge 14 in the crucible 13. The envelope 6 is then replaced in its former sealed position and is again evacuated to a pressure less than 10 microns of mercury.

At the beginning of the melting operation the coil 20 is lowered to a position such that the maximum heating is applied to thetungsten block 16, which in turn, heats the mold 15 until these parts have reached a temperature above the melting point of uranium. Preheating of the mold 15 and its support 16 has been found to be effective to prevent the molten metal from freezing immediately on flowing into and striking the cold mold. Quick order to drive out any entrapped or occluded gases, or

other vaporizable material.

According to my preferred procedure, the apparatus shown in Fig. l is assembled with a uranium button 1 in place in the crucible 2, as described hereinbefore. The envelope 6 is hermetically sealed to the base 5 by suitable means, such as an Apiezon sealing wax (sold in several grades by James G. Biddle Company, Philadelphia, Pennsylvania, and well known to the trade. See Procedures in Experimental Physics by Strong, Prentice Hall, Inc., 1943), and is then evacuated by means of a vacuum pump connected to pipe 24 until the pressure within the envelope 6 is less than 10 microns of mercury. The pressure should be held to 10 microns or less throughout the melting process in order to prevent contamination of the melt. ated, the coil 20 is connected to the high frequency current supply to heat the uranium button 1 by induction.

As the temperature of the button is raised above the melting point of uranium, fused metal collects in the When the chamber 6 has been evacubottom of the crucible 2, and the residue from the bottoms floats on the surface of the melt. The heating is continued until substantially all of the metal is run out of the buttons to form a pool 10, as shown in Fig. 2, with the residue concentrated in a mass 11 floating on the surface of the pool. Heating is then discontinued, and the mass is allowed to cool, assisted by introduction of an inert gas such as helium or argon through the pipe 25. The inert gas pressure inside the envelope 6 may conveniently be raised to from one-half to three-quarters freezing causes an undesirable uneven bottom surface on the casting and hence should be avoided. The massive tungsten block 16 retains enough heat so that the bottom of the mold remains hot when the heating coil 20 is moved upward in the next step. As the temperature of the semi-refined metal 12 is raised above its melting point,

the metal flows through the openings in the bridge 14 and into the mold 15. The slag 11 is held on the bridge 14, and substantially complete separation of the metal from the slag is achieved. When the metalhas ceased to flow,

the coil 20 is raised until its lower end is approximately on a level with the mid-point of the mold 15. The heating is continued in this position for from 5 to 10 minutes, after which the coil 20 is disconnected. This final heating is necessary in order to insure that the casting will solidify or freeze progressively from the bottom to the top. Castings that freeze downward from the top surface are apt to contain undesirable voids or gas pockets.

The vacuum in the envelope 6 is, of course, maintained as before throughout the melting operation, following which the inert gas is again admitted to cool the metal.

Uranium cast by the herein described method will form a solid casting free from voids, and the metal will be ductile and easily worked as a result of the high degree of purity obtained. This may be accomplished with a loss to the'slag of as little as 10 percent of the uranium in the original button by virtue of the concentration of the slag during the premelting operation first described.

Although a preferred apparatus and method have been described, it will be understood that variations and modifications in both the method and apparatus may be made without departing from the scope of the invention as defined by the appended claims. For example, the electrical conducting coil employed inside the glass envelope of an atmosphere at the beginning of the cooling operation, and more gas may be added as the pressure drops with cooling. The gas conduits the heat from the metal in the crucible and from the adjacent equipment to the glass envelope 6, and a blast of air from the fan 21 may be directed at the outer surface of the envelope to hasten 5 the transfer of heat away from the apparatus. In addition to speeding the process, such cooling aids in controlling vaporized impurities given off from the button that may not have been entirely removed from the glass envelope by the evacuating apparatus. Removal of such vaporized impurities reduces the danger of having them recombine with the hot metal. The effect is both to condense some of these impurities on the interior of the cooled glass envelope and to reduce the vapor pressure of the impurities. I

When the uranium has cooled to a temperature at which and around the crucible by Meister in Serial No. 533,112 may be employed during the premelting operation of the present invention if the impurities in the original uranium button are so high as to make themelting diflicult by direct induction heating .of the button. This is totally unnecessary for the second melting operation described herein because of the concentration of the impurities adjacent the surface of the uranium mass with a consequent improvement in the conductivity of the button. Also, if desired, the crucibles 2 and 13 may be shielded in any desired manner to reduce the loss of heating by radiation from the uranium mass during both the first and second melting operations. Many other modifications of the ap-' paratus shown herein. may obviously be made in carrying out the steps of the process, which is not limited. to the use of any particular apparatus.

I claim:

1. The method of refining uranium comprising the steps of melting a mass of impure uranium to concentrate nonvolatile impurities upon the surface of said uranium, allowing themass to solidify .with the impurities so concentrated, raising the temperature of the mass abovethe melting point of uranium, and straining the melted uranium, thereby separating solid impurities therefrom, while protecting said uranium from reactive gases.

2. The method of refining uranium comprising the steps of melting a mass of impure uranium to concentrate non-volatile impurities upon the surface of said uranium, allowing the mass to solidify with the impurities so concentrated, raising the temperature of the mass above the melting point of uranium and straining the melted uranium, thereby separating solid impurities therefrom, casting the refined uranium in a mold, and maintaining said uranium out of contact with reactive gases.

3. The method of refining uranium comprising the steps of melting a mass of uranium in a confined space to concentrate non-volatile impurities upon the surface of said uranium, causing the mass to solidify with the impurities so concentrated, raising the temperature of the mass above the melting point of uranium, straining the melted uranium, thereby separating solid impurities therefrom, protecting said uranium from reactive gases.

4. The method of refining uranium comprising the steps of melting a mass of uranium in a confined space to concentrate non-volatile impurities as a slag upon the surface of the molten mass, causing the mass to solidify with the impurities so concentrated, raising the temperature of the mass above the melting point of uranium, straining the melted uranium, thereby separating impurities therefrom, while protecting said uranium from reactive gases.

5. The method of refining uranium comprising the steps of melting a mass of uranium in a confined space to concentrate non-volatile impurities as a slag upon the surface of the molten mass, causing the mass to solidify with the impurities so concentrated, raising the temperature of the mass above the melting point of uranium, straining the melted uranium, thereby separating solid impurities therefrom, casting the refined uranium in a mold, and maintaining said uranium out of contact with reactive gases.

6. The method of refining uranium comprising the steps of melting a mass of uranium in a confined space and in a vacuum of approximately microns of mercury to concentrate non-volatile impurities as a slag upon the surface of the molten mass, causing the mass to solidify in the substantial absence of oxidizing gases, and with the impurities concentrated in a localized part of the mass, raising the temperature of the mass above the melting point of uranium in a vacuum of approximately 10 microns of mercury, and straining the melted uranium, thereby separating solid impurities therefrom, while maintaining the uranium in said vacuum.

7. The method of refining uranium comprising the steps of melting a mass of uranium in a confined space and in a vacuum of approximately 10 microns of mercury to concentrate non-volatile impurities as a slag upon the surface of the molten mass, causing the mass to solidify in said confined space in the substantial absence of oxidizing gases, raising the temperature of the mass above the melting point of uranium in a vacuum of approximately 10 microns, straining the melted uranium to separate the molten uranium from said non-volatile impurities and casting the refined uranium in a mold while maintaining the uranium in said vacuum.

8. The method of refining uranium comprising the steps of heating a mass of uranium containing impurities in a crucible to the melting point of uranium in a vacuum of approximately 10 microns of mercury until at least a part of the volatile impurities are liberated and the non-volatilized impurities are largely concentrated as a slag upon the surface of the molten mass, cooling the mass With the assistance of a non-oxidizing gas to a temperature at which the uranium metal will not readily oxidize in air, transferring the solidified mass to a crucible having a bottom drain, reheating the mass to the melting point of uranium in a vacuum of approximately 10 microns of mercury, and causing the molten uranium to flow from the concentrated non-volatilized impurities and through said drain, while maintaining the uranium in said vacuum.

9. The method of recovering uranium in a substantially pure coalescent form from a coherent button of compressed impure uranium powder comprising melting said button in a crucible in a vacuum of approximately 10 microns of mercury to concentrate non-volatile impurities as a slag upon the surface of the molten mass, causing the mass to solidify in said crucible in the substantial absence of oxidizing gases, remelting the mass in a crucible having a bottom drain in a vacuum of approximately 10 microns of mercury while causing the molten uranium to flow from said slag and through said drain.

10. The method of recovering uranium in a substantially pure coalescent form from a coherent button of compressed impure uranium powder comprising melting said button in a crucible in a vacuum of approximately 10 microns of mercury to concentrate non-volatile impurities as slag upon the surface of the molten mass, causing the mass to solidify in said crucible in the substantial absence of oxidizing gases, remelting the mass in a crucible having a bottom drain in a vacuum of approximately 10 microns of mercury While causing the molten uranium to flow from said slag and through said drain into an evacuated mold, and cooling the purified uranium in the substantial absence of oxidizing gases to a temperature at which uranium metal will not readily oxidize in air.

References Cited in the file of this patent UNITED STATES PATENTS 1,106,384 Hughes Aug. 11, 1914 1,292,582 Coulson Jan. 28, 1919 1,752,474 Anderson Apr. 1, 1930 1,814,719 Marden et al. July 14, 1931 

1. THE METHOD OF REFINING URANIUM COMPRISING THE STEPS OF MELTING A MASS OF IMPURE URANIUM TO CONCENTRATE NONVOLATILE IMPURITIES UPON THE SURFACE OF SAID URANIUM, ALLOWING THE MASS TO SOLIDIFY WITH THE IMPURITIES SO CONCENTRATED, RAISING THE TEMPERATURE OF THE MASS ABOVE THE MELTING POINT OF URANIUM, AND STRAINING THE MELTED URANIUM, THEREBY SEPARATING SOLID IMPURITIES THEREFROM, WHILE PROTECTING SAID URANIUM FROM REACTIVE GASES. 